The Prior Art
Touch input devices are devices of the type having a relatively flat screen or plate, the space in front of which is traversed by a plurality of beams of radiant energy. The beams are generally arranged to define a plane parallel to the surface of the plate. When a point on the plate is touched with a finger or stylus, light beams are interrupted, and signals indicating which light beams have been interrupted are used to develop signals corresponding to the X and Y coordinates of the point on the plate which has been touched. Typically, infrared sources such as LED's or the like are employed to produce the light beams, and phototransistors or other IR detectors are employed to detect the light beams, with the LED's mounted along one edge of the panel, and the phototransistors mounted along the opposite edge, in line with the beams produced by the LED's. Since touch panels are typically used in lighted rooms, having relatively high ambient light level conditions, the phototransistors are subject to variations in the ambient light level, and their outputs depend not only on the light energy emitted from their respective LED's, but also on the ambient light conditions. Typically, a threshold level must be selected for the phototransistors, to enable them to detect the radiant energy produced by their associated LED's, and not any ambient light conditions. Any fixed threshold level is necessarily a compromise, which degrades the performance of the apparatus, and may also render the apparatus unusable if the ambient lighting conditions are changed markedly.